1. Field of the Invention
The present invention relates to methods and apparatus for downlink Physical Hybrid Automatic Repeat-reQuest (HARQ) Indicator Channel (PHICH) mapping and channelization in a communication system.
2. Description of the Related Art
This application, pursuant to 37 C.F.R. §1.57, incorporates by reference the following publications:
[1] 3GPP LTE 36.211, version 8.2.0;
[2] R1-073099, “Downlink ACK/NACK Transmit Diversity”, Samsung;
[3] R1-081036, “Investigation on Radio Resource Indication for PHICH in E-UTRA Downlink”, NTT DoCoMo, Fujitsu, Mitsubishi Electric, Sharp, Toshiba Corporation;
[4] R1-081132, “Orthogonal Sequence Definition for PHICH”, Qualcomm;
[5] R1-081071, “Way Forward on PHICH Allocation”, Nokia, NSN, Samsung, LGE, NTT DoCoMo, Nortel, NEC, Motorola, Panasonic, Ericsson, Qualcomm;
[6] 3GPP LTE 36.213, version 8.1.0; and
[7] R1-081063, “Text Proposal for PHICH to RE mapping with cell ID”, LGE, Samsung, Nortel, Panasonic, Motorola, Ericsson, Nokia, NSN, Qualcomm.
Telecommunication enables transmission of data over a distance for the purpose of communication between a transmitter and a receiver. The data is usually carried by radio waves and is transmitted using a limited transmission resource. That is, radio waves are transmitted over a period of time using a limited frequency range.
In Third (3rd) Generation Partnership Project Long Term Evolution (3GPP LTE) systems, certain resource elements are allocated for control signal transmission. Therefore, the data symbols may be mapped into the resource elements that are not allocated for control signal transmission. Each data transmission carries information bits of one or multiple transport blocks. When a transport block is larger than the largest code block size, the information bits in a transport block may be segmented into multiple code blocks. The process of dividing the information bits in a transport block into multiple code blocks is called code block segmentation. Due to the limited selection of code block sizes and the attempt to maximize packing efficiency during the code block segmentation, the multiple code blocks of a transport block may have different sizes. Each code block will be encoded, interleaved, rate matched, and modulated. Therefore, the data symbols for a transmission may consist of modulation symbols of multiple code blocks.
A downlink physical channel corresponds to a set of resource elements carrying information originating from higher layers. Physical Hybrid Automatic Repeat-reQuest (HARQ) Indicator Channel (PHICH) is a downlink acknowledgement channel that are used by a base station (enhanced Node B, i.e., eNB) to acknowledge (ACK) or negatively acknowledge (NAK) uplink transmissions by a plurality of units user equipment (UE). In the downlink transmission, Physical Hybrid Automatic Repeat ReQuest (HARQ) Indicator Channel (PHICH) is defined for carrying the HARQ acknowledgement or negative acknowledgement signals. According to the third Generation Partnership Project 3GPP LTE standard [1], multiple PHICH resources mapped to the same set of resource elements constitute a PHICH group, where the PHICH resources within the same PHICH group are separated through different orthogonal sequences. A PHICH resource is identified by an index pair
      (                  n        PHICH        group            ,              n        PHICH        seq              )    ,where
  n  PHICH  groupis the PHICH group index, and
  n  PHICH  seqis the orthogonal sequence index used for the PHICH resource within the PHICH group. The number of symbols within a PHICH group is the same as the one in a PHICH sequence, and a PHICH group is a sequence of twelve (12) symbols. Each quadruple (four (4) symbols) in a PHICH group is mapped to a resource-element group. Thus the twelve (12) symbols in a PHICH group are mapped to three resource-element groups.
Currently, a few methods were proposed to allocate the PHICH. In 3GPP contribution R1-073099 [2], I/Q domain multiplexing is proposed for DL PHICHs in order to increase the ACK/NACK multiplexing capacity. As pointed out in the same contribution [2], the I/Q multiplexing results in an I/Q leakage problem in the case of severe channel estimation error at the UE.
In contribution R1-081132 [4], it was stated that alternative spreading sequences for PHICH can alleviate the I/Q leakage problem. In fact, the new approach in contribution R1-081132 [4], however, does not alleviate the I/Q leakage problem but only gives the same performance as the original approach.
In a way forward on PHICH allocation [3], it is agreed to link PHICH groups to PRB indices. This implies that a UE will have only a single PHICH resource index pair
  (            n      PHICH      group        ,          n      PHICH      seq        )for the three repetitions. Suppose that two UEs are assigned to index pairs
      (                  n        PHICH        group            ,              n        PHICH                  seq          ,          1                      )    ⁢          ⁢  and  ⁢          ⁢      (                  n        PHICH        group            ,              n        PHICH                  seq          ,          2                      )  respectively. We further suppose that
      n    PHICH          seq      ,      1        ⁢          ⁢  and  ⁢          ⁢      n    PHICH          seq      ,      2      corresponds to Walsh codes W and jW, respectively. Then, a UE located closer to eNodeB may experience I/Q leakage problem during all the three PHICH repetitions.